Silicon electro-optic modulators play an increasing role in the field of optic communication owing to their process compatibility with CMOS technology. Standard silicon electro-optic modulators typically employ MOS capacitors or reverse-biased PN junctions to achieve a large modulation bandwidth even up to 30 GHz.
The modulation depth, i.e., the total phase change of the modulator, is proportional to the length of the modulator, thus a longer device is preferred in the point of modulation depth. For such a long modulator, a properly designed microwave traveling electrode such as a coplanar waveguide (CPW) or a coplanar strip line (CS) is required for high-speed modulation. Generally, a CPW electrode rather than a CS electrode is chosen as shown in FIG. 1 because of its ease for test and compatibility with legacy driver electrode scheme. The active phase shifter, i.e., the PN junctions or MOS capacitors, is located in one slot of the CPW and connected to one of the two ground metals (G) and the signal metal (S) by properly impurity-doped regions. This common electrode scheme constructs an asymmetric CPW because only one slot is loaded with capacitance from the electrically-connected phase shifter. There are several electromagnetic field modes in an asymmetric CPW such as the CPW even mode, odd mode, and the surface wave-like mode, with different microwave impedance for each. The CPW even mode is the needed one and excited by the driver, and the electrode is terminated by terminator with impedance equal to that of the mode. However, such an asymmetric CPW is prone to the microwave mode conversion when the excited CPW even mode is propagated along the CPW. The converted mode has impedance different from that of the CPW even mode and the terminator, thus a microwave reflection will occur due to impedance mismatch and limit the bandwidth of the modulator. FIG. 2 shows the electro-optic response of a modulator using such an asymmetric CPW electrode. The dip around 11 GHz in the response curve is caused by the impedance mismatch between terminator and the modes other than CPW even mode.
Generally, a silicon electro-optic modulator is designed with high capacitance per unit length to obtain modulation efficiency as high as possible. However, the high capacitance per unit length usually gives rise to low microwave impedance of the order 20˜30 ohm. Such low impedance makes the impedance matching to the modulator driver difficult, and a specially designed driver with output impedance much lower than standard 50 ohm is required. In the meantime, the low microwave impedance of the modulator and driver increases the RF power consumption of the transmitter employing the modulator.